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  <title>Document</title>
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<body>
  <script>
    class ListNode {
      constructor(val) {
        this.val = val
        this.next = null
      }
    }
    let list = new ListNode('a')
    list.add('b')
    list.add('c')
    console.log(list);
    if(!list) {
      console.log(11)
    }else{
      console.log(22)

    }
  </script>
</body>
</html>